Waveform Synthesis Research Articles

Towards GW-Scale Isolated Attosecond Pulse Far beyond Carbon K-Edge Driven by Mid-Infrared Waveform Synthesizer

We discuss the efficient generation of intense “water window” (0.28–0.54 keV) isolated attosecond pulses (IAPs) using a mid-infrared (MIR) waveform synthesizer. Our numerical simulations clearly indicate that not only a longer-wavelength driving laser but also a weak control pulse in the waveform synthesizer helps extend the continuum cutoff region and reduce the temporal chirp of IAPs in high-order harmonic generation (HHG). This insight indicates that a single-cycle laser field is not an optimum waveform for generating the shortest IAP from the veiwpoints of reducing the attochirp and increasing the efficiency of HHG. By combining a waveform synthesizer technology and a 100 mJ MIR femtosecond pulse based on a dual-chirped optical parametric amplification (DC-OPA) method, a gigawatt-scale IAP (55 as with 10 nJ order) in the water window region can be generated even without attochirp compensation. The MIR waveform synthesizer is highly beneficial for generating a shorter IAP duration in the soft X-ray region because there are no suitable transparent dispersive materials that can be used for compressing the attochirp.

Control of Rydberg-state population with realistic femtosecond laser pulses

We investigate computationally a method for ultrafast preparation of alkali metal atoms in their Rydberg states using a three-dimensional model potential in the single active electron approximation. By optimizing laser pulse shapes that can be generated with modern waveform synthesizers, we propose pulses for controlling the population transfer from the ground state to a preselected set of Rydberg states. Dynamical processes under the optimized pulses are shown to be much more complicated than in the traditional optical two-photon preparation of Rydberg states.

Wavefront analysis of a white-light supercontinuum.

The wavefront quality of white-light supercontinuum is crucial for applications like waveform synthesis or imaging. It has been here generated by 1030 nm centered sub-picosecond pulses in YAG and characterized with a Shack-Hartmann wavefront sensor across different parts of the spectrum. It shows a good wavefront quality of λ/11 and little dependence on the wavelength of the supercontinuum. The wavefront deformations are transferred from the driver laser wavefront to the supercontinuum independently of the wavelength.

The design of a wide bandwidth time marker generator.

The Time Marker Generator (TMG) is an important instrument that is used to calibrate the time base of an oscilloscope. If a direct digital frequency synthesizer is used to generate the necessary waveforms, there is serious distortion in the generated square wave when the ratio of the sampling frequency to the output frequency is non-integer. In addition, the look-up table (LUT) in the direct digital waveform synthesizer needs a large storage capacity to generate a narrow triangular wave at a low output frequency. This paper proposes a design that will generate the time marker whose samples are synthesized by real-time calculation instead of storing them in a LUT. We also propose an M waveform data synthesizer with a parallel structure (M-WDSPS) to reduce the operating clock frequency of the field programmable gate array (FPGA). We built a 4-WDSPS TMG based on this design and reduced the required clock frequency from 1 GHz to 250 MHz, thus making the implementation possible in an FPGA. Our proposed TMG design can provide square wave, pulse wave, narrow triangular wave, and linear triangular wave with different amplitudes in two operating modes: normal mode and highlight mode. The TMG we constructed can provide the time marker with an output frequency range from 10 mHz to 111 MHz, a rising/falling edge time lower than 1 ns, regardless of whether the output frequency is low or high, and a frequency stability lower than 0.1 ppm.

Ultrashort pulse synthesis for energy concentration control in nanostructures

A waveform synthesis technique is introduced and applied to the femtosecond pulse excitation of plasmonic nanoantennas for temporal and spatial energy concentration control. The waveform synthesis process is based on phase and polarization shaping and an understanding of the electromagnetic response of the nanostructure. Linear and radial nano-dipole arrays are analyzed before the log-periodic toothed nanoantenna is investigated as a nanostructure capable of combining the benefits of the nano-dipole arrays. The consistent superiority of the log-periodic toothed nanoantenna is established by comparing its electromagnetic response to that of the radial nano-dipole array using a variety of synthesized excitation waveforms.

Impact of the latest generation of Josephson voltage standards in ac and dc electric metrology

For decades, the quantum behavior of Josephson junctions has been employed as intrinsic standards for voltage metrology. Conventional dc Josephson voltage standards have been the primary standards for voltage, programmable Josephson voltage standards have been implemented in calibration services and precision measurements, such as the Planck constant, and Josephson arbitrary waveform synthesizers have been employed in ac voltage calibrations and precision measurements of the Boltzmann constant. With the anticipated redefinition of the Système International d’Unités, all types of Josephson voltage standards will become intrinsic standards and equivalent realizations of the unit volt. Here we review the state-of-the art performance, best practices, and current impact of these systems for various applications, with an emphasis on ac voltage metrology. We explain the limitations of each system, especially regarding the many potential systematic errors that affect their accuracy and performance for specific applications.

Mel-Cepstrum-Based Quantization Noise Shaping Applied to Neural-Network-Based Speech Waveform Synthesis

This paper presents a mel-cepstrum-based quantization noise shaping method for improving the quality of synthetic speech generated by neural-network-based speech waveform synthesis systems. Since mel-cepstral coefficients closely match the characteristics of human auditory perception, the proposed method effectively masks the white noise introduced by the quantization typically used in neural-network-based speech waveform synthesis systems. The paper also describes a computationally efficient implementation of the proposed method using the structure of the mel-log spectrum approximation filter. Experiments using the WaveNet generative model, which is a state-of-the-art model for neural-network-based speech waveform synthesis, showed that speech quality is significantly improved by the proposed method.

PSO based adaptive narrowband ANC algorithm without the use of synchronization signal and secondary path estimate

Abstract Narrowband active noise control (ANC) conventionally uses the waveform synthesis method in which a synchronization signal is essential. In addition to this, to update the parameters of the controller, prior estimation of the secondary path is required. Any mismatch in the frequency of the synchronization signal and the secondary path estimate compared to actual leads to a degraded performance in noise control. In this paper, a new type of narrowband ANC algorithm is proposed which requires neither the synchronization signal nor the secondary path estimate for controller tuning. Therefore, the algorithm is free from the effect of the change in the source frequency and the secondary path transfer function. The proposed adaptive algorithm is based on a non-gradient type optimization algorithm named as particle swarm optimization (PSO) algorithm. Through exhaustive computer based simulation, it has been demonstrated that the algorithm is not only capable to control a monotone noise but also efficiently control a multi-tone noise. The algorithm, due to its adaptive nature, is also capable of tracking the change in amplitude and frequency of the noise as well as, the change in the characteristics of primary and secondary paths.

Ultimate waveform reproducibility of extreme-ultraviolet pulses by high-harmonic generation in quartz

Optical waveforms of light reproducible with subcycle precision underlie applications of lasers in ultrafast spectroscopies, quantum control of matter and light-based signal processing. Nonlinear upconversion of optical pulses via high-harmonic generation in gas media extends these capabilities to the extreme ultraviolet (EUV). However, the waveform reproducibility of the generated EUV pulses in gases is inherently sensitive to intensity and phase fluctuations of the driving field. We used photoelectron interferometry to study the effects of intensity and carrier-envelope phase of an intense single-cycle optical pulse on the field waveform of EUV pulses generated in quartz nanofilms, and contrasted the results with those obtained in gas argon. The EUV waveforms generated in quartz were found to be virtually immune to the intensity and phase of the driving field, implying a non-recollisional character of the underlying emission mechanism. Waveform-sensitive photonic applications and precision measurements of fundamental processes in optics will benefit from these findings. High-harmonic generation in quartz offers immunity to the extreme-ultraviolet waveform against the intensity and phase noise of the driving laser pulse, extending precision waveform synthesis to the extreme ultraviolet.

Jitter Sensitivity Analysis of the Superconducting Josephson Arbitrary Waveform Synthesizer.

We present the first jitter sensitivity analysis of a superconducting voltage reference waveform synthesizer with fundamentally accurate output pulses. Successful deployment of a reference waveform source at microwave frequencies will represent a new paradigm for radio frequency metrology. The programmable waveform synthesizer considered in this paper contains a 1.5 bit delta-sigma digital-to-analog converter (DAC) with a sampling frequency of 28 GHz. We quantify the impact of random and deterministic output pulse position jitter (PPJ) on: 1) the amplitude accuracy of the output fundamental tone and 2) the in-band signal-to-noise and distortion ratio (SNDR). The superconducting DAC features a complete lack of output pulsewidth jitter, and random PPJ up to 200 fs rms has a negligible impact on accuracy and SNDR for synthesized tones up to 1 GHz. However, application of nonzero dc bias current is shown to produce deterministic PPJ of up to 5 ps, which, in turn, is shown to degrade the in-band SNDR by 30 dB at 1 GHz unless eliminated with techniques discussed in this paper. We verify the predicted effects of random and deterministic PPJ with simulations in the range of 100 kHz-1 GHz and with experiments in the range of 100 kHz-3 MHz.

Attosecond relative delay measurement using transient-grating frequency-resolved optical grating

The accurate and precise controlling of the attosecond time delay between the sub-pulses within a hundredth of an optical cycle is the key ingredient for the sophisticated custom-tailored coherent waveform synthesizer. The attosecond delay control technique commonly experiences the “complete” characterization of the ultrashort sub-cycle pulses, which includes the spatiotemporal pulse characterization of the synthesized waveform and the attosecond relative delay between the parent pulses. In this work, the relative time delay between spectrally separated ultrashort parent pulses is characterized in an interferometer scheme with a background-free transient-grating frequency-resolved optical grating (TG-FROG). The TG-FROG geometry accurately measures the full time-dependent intensity and phase of ultrashort laser pulses in a wide range of regime (from ultraviolet to infrared) and offers significant advantages over other nonlinear-optical processes geometries (i.e., the polarization-gate-FROG, the self-diffraction-FROG, the second-harmonic generation-FROG and the third-harmonic-generation-FROG). The attosecond measurement accuracy is achieved for the first time, to the best of our knowledge. In this experiment, the output of a carrier-envelope-phase-stable Ti:sapphire amplifier (sub-30-fs, over-1-mJ, 1 kHz) is spectrally broadened in a neon-filled hollow-core fiber with an inner diameter of 250μm. The transmission through the pressure-gradient hollow-core fiber results in an mJ-level octave-spanning whitelight supercontinuum, supporting a sub-3-fs Fourier transform-limited pulse. The supercontinuum is spectrally divided into two parent pulses by using a dichroic mirror. The sub-pulses are individually compressed by the custom-designed double-chirped mirrors and wedge pairs. The short and long wavelength pulses are separately compressed in few-cycle regime, yielding pulses with 6.7 fs and 9.8 fs, respectively. This technique overcomes the bottlenecks in the traditional delay measurement and should be applicable for many ultra-broadband pulse characterizations with extremely simple and alignment-free delay control device used. Furthermore, this new method will be easily adapted for the ultra-broadband two-dimensional electronic spectroscopy, the advanced temporal cloaking, and the field of sub-cycle arbitrary coherent waveform synthesizer for controlling strong-field interactions in atoms, molecules, solids, and nanostructures. We foresee that in the near future this novel technology will be very attractive for various applications in the next-generation light sources such as the Synergetic Extreme Condition User Facility in Beijing, China.

Direct temporal shaping of terahertz light pulses

Active terahertz (THz) waveform synthesis is desirable for a broad range of applications including high speed wireless communications at Tb/s speeds and coherent manipulation of quantum systems driven by engineered light fields. In this work, we demonstrate an all-optical, fully reconfigurable platform for direct and arbitrary temporal shaping of broadband THz light pulses. The technique is based on an array of line photoexcitations of charge carriers within an otherwise homogeneous semiconductor embedded within a parallel plate waveguide. The spatially periodic charge distributions locally modulate the THz dielectric function, mapping each photoexcited line in the array directly to a reflected single-cycle THz pulse. By tuning the spatial distribution of the line excitations, arbitrarily shaped THz pulse sequences can be created from a single THz input pulse with relatively little intrinsic loss. We demonstrate synthesis of multi-cycle THz waveforms, 8-bit digital pulse sequences, and THz frequency combs with control over amplitude, central frequency, and linewidth.

Self-starting harmonic frequency comb generation in a quantum cascade laser

Optical frequency combs establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications. Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis and for generation of THz tones of high spectral purity in the future wireless communication networks. We demonstrate for the first time self-starting harmonic frequency comb generation with a THz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of the population inversion in the laser. The mode spacing of the harmonic comb is shown to be uniform to within $5\times 10^{-12}$ parts of the central frequency using multiheterodyne self-detection. This new harmonic comb state extends the range of applications of quantum cascade laser frequency combs.

Multibeam synthesis of high-power subcycle field waveforms

We identify physical scenarios whereby high-peak-power subcycle attosecond field waveforms can be synthesized by coherently combining a multibeam high-order harmonic output generated by a laser driver consisting of a pair of few-cycle pulses with different carrier frequencies. With the relative amplitudes, phases, and group delays of these driver pulses carefully adjusted in each of the driver beams toward confining the recollisions of highest-ponderomotive-energy electrons to an extremely short time gate within a fraction of the driver field cycle, the phase-matched multibeam high-harmonic output can be tailored to yield an intense isolated subgigawatt sub-10-attosecond field waveform. As a general tendency, propagation effects are shown to limit the minimum pulse width of the multibeam high-harmonic output. Still, with appropriate optimization of the gas pressure and the beam geometry, $\ensuremath{\approx}10$-as field waveforms can be synthesized at the expense of one to two orders of magnitude of the output radiation energy.

Packaging and Demonstration of Optical-Fiber-Coupled Photodiode Array for Operation at 4 K

An optoelectronic module for operation at 4 K is developed, intended for use in pulse-driven Josephson arbitrary waveform synthesizer. Multiple InP/InGaAs photodiodes with customized optical fiber assembly were assembled on a single silicon carrier. Photodiodes were flip-chip bonded on the carrier using Au stud bumps, and laser-cut silicon fixtures were aligned and adhesively bonded to the carrier in order to attach pigtailed borosilicate ferrules. Optical simulations were performed to estimate the tolerance for fiber-chip misalignment. The photodiodes were bonded with an average misalignment of $8~\mu \text{m}$ , while the misalignment between the silicon fixture and the photodiodes after bonding was $13~\mu \text{m}$ . The electrical response to continuous-wave laser inputs was measured at room temperature and at 4 K by direct immersion in liquid helium. The results show that the assembly technique could facilitate a stable and efficient optical coupling. The individual photodiodes were able to deliver currents up to 12 mA at room temperature and 7 mA at 4 K.

Keysight 3458A Noise Performance in DCV Sampling Mode

This paper describes an evaluation of the Keysight 3458A digital multimeter (DMM) noise performance. A DMM noise model is given in closed form for all ranges and integrating analog-to-digital converter (IADC) aperture times. When the DMM is used for sampling, different sources of noise contribute to the total noise of the sampled signal. Their combined effect was measured using both short-circuited input terminals and spectrally pure Josephson arbitrary waveform synthesizer. Both white noise and ${1/f}$ noise were evaluated. The results confirmed that the internal IADC can be used to measure DMM input referred noise when the IADC quantization noise is smaller than the instrument front-end analog circuitry-related noise. Further, it is shown that the DMM noise can be effectively measured with short-circuited input terminals when using double integer output format, but not always when using single integer output format. The manufacturer specifications were confirmed on all but the 100-V range.

A Precision Microvolt-Synthesizer Based on a Pulse-Driven Josephson Voltage Standard

In this paper, we describe the development of a precision microvolt-synthesizer, which could be part of a system that enables much larger amplitudes up to 1 V. The synthesizer is based on a pulse-driven Josephson arbitrary waveform synthesizer covering wide voltage ranges from nanovolt to millivolt and wide frequency ranges from dc to audio frequencies, respectively. We demonstrate 1.7-mA-wide current margins for the pulse-driven system that make automated calibration of typical null detectors, such as nanovoltmeters or lock-in amplifiers, easy. Uncertainties at the nanovolt level can be achieved within short measurement times, depending on the instruments being calibrated.

Optimal control of photoelectron emission by realistic waveforms

Recent experimental techniques in multicolor waveform synthesis allow the temporal shaping of strong femtosecond laser pulses with applications in the control of quantum mechanical processes in atoms, molecules, and nanostructures. Prediction of the shapes of the optimal waveforms can be done computationally using quantum optimal control theory. In this work we demonstrate the control of above-threshold photoemission of one-dimensional hydrogen model with pulses feasible for experimental waveform synthesis. By mixing different spectral channels and thus lowering the intensity requirements for individual channels, the resulting optimal pulses can extend the cutoff energies by at least up to 50% and bring up the electron yield by several orders of magnitude. Insights into the electron dynamics for optimized photoelectron emission are obtained with a semiclassical two-step model.

Rapidly reconfigurable high-fidelity optical arbitrary waveform generation in heterogeneous photonic integrated circuits.

This paper demonstrates rapidly reconfigurable, high-fidelity optical arbitrary waveform generation (OAWG) in a heterogeneous photonic integrated circuit (PIC). The heterogeneous PIC combines advantages of high-speed indium phosphide (InP) modulators and low-loss, high-contrast silicon nitride (Si3N4) arrayed waveguide gratings (AWGs) so that high-fidelity optical waveform syntheses with rapid waveform updates are possible. The generated optical waveforms spanned a 160 GHz spectral bandwidth starting from an optical frequency comb consisting of eight comb lines separated by 20 GHz channel spacing. The Error Vector Magnitude (EVM) values of the generated waveforms were approximately 16.4%. The OAWG module can rapidly and arbitrarily reconfigure waveforms upon every pulse arriving at 2 ns repetition time. The result of this work indicates the feasibility of truly dynamic optical arbitrary waveform generation where the reconfiguration rate or the modulator bandwidth must exceed the channel spacing of the AWG and the optical frequency comb.

Amplifier-in-the-Loop Adaptive Radar Waveform Synthesis

Adaptive algorithms having the capacity to perform real-time optimization in response to a dynamic environment will be necessary for the development of a cognitive radar technology. This paper details an amplifier-in-the-loop algorithm to synthesize radar waveforms which are optimized for desired ambiguity function characteristics in a monostatic radar system, with constraints on the peak-to-average power ratio and spectrum of the waveform. The algorithm uses alternating projections to search for a waveform whose ambiguity function best approximates the goal for a volume-based constraint in desired regions of the range–Doppler plane. Results are shown for both simulation and measurement cases, both with and without the effects of a nonlinear amplifier. This waveform synthesis method is expected to be useful in real-time adaptive radar systems.